Client/Server Environment
In a client/server environment, a server holds it's applications, resources, and files.  Other computers in the network can access resources stored in the server

• The server is always ON. If the server is turned OFF, access to clients is not available.
  
  


• Client/server networks can have many servers. The application server, Web Server, Mail Server, DNS Server, Proxy Server, DHCP server, etc.

The client/ Server model provides better security.
To access the network, user must be authenticated with a username and a password.

Advantages are security, centralized backup, Intranet capability, Internet monitoring

Web Server

Web servers deliver Web pages. 
Web servers use HTTP and HTTPS protocols
Every Web server has an IP address and possibly a domain name. 
Web servers hold static web pages and dynamic business applications.
Web Server Software: Apache, Internet Information Server


The browser communicates with DNS to translate "www.kpauta.com" into an IP Address. Then the browser sent a GET request to the server, asking for the file "http://www.kaputa.com/index.htm." The server sent the HTML text for the Web page to the browser. The browser renders the HTML text.

A data center for web services
Observe how load balancing is done and database clustering using multiple servers.
The requests are shared between servers.
 If one server fails the other server takes on requests. 

Mail Server 
Mail servers provide e-mail service to users. 

Mail server has a database of user accounts, mail storage, and modules to handle the transfer of messages to and from other mail servers and email clients.

Proxy Server
Proxy is a  computer acting as an agent between user and another server or web service. 


Organizations use proxy servers to 

• Cache most accessed content


• Limit access to the internet


• Monitor internet activities


• Proxy servers can help anonymous browsing (Privacy)

DNS Server
A DNS server provides Domain name to IP address resolution on the internet.

DNS Root Servers

The DNS is a distributed system. It has 13 root servers with a database of domain names and addresses. 

• Domains registered are stored in root DNS servers.


• DNS servers communicate with each other using private network protocols. 


• All DNS servers are organized in a hierarchy.


• Lower-level DNS servers maintain only certain parts of the overall database.


• DNS is based on the client/server network architecture.

1. Your Web browser functions as a DNS client and make web requests to your Internet provider's DNS servers.


2. When a DNS server receives a request, it checks it's a database.


3. If the domain not found, the server passes the request to a higher-level DNS.


4. The request finally reach a server that has the matching name and IP address in its database (root DNS),


5. Then the response flows back through the chain of DNS servers to your client.
   
   

Dynamic Host Configuration Protocol (DHCP)
DHCP automatically provides dynamic IP address to hosts in a network.

DHCP allows devices needing an IP address to request one when they are starting up. 
DHCP is a used when limited IPs are avialable to assign to devices.
DHCP is limited to a specific local area network (LAN).
DHCP is not a secure protocol.